So, an animatic for the Season 5 finale showed that Starlight Glimmer is going to return, right after Twilight gives a big speech about how all the Mane 6 got their cutie marks because of Rainbow Dash's first Sonic Rainboom.

So, what are your theories on what her plans for revenge are and what will happen this episode?

John Sullivan suggested, and I think I agree with him, that Starlight might travel back in time to keep other ponies from getting their cutie marks. Seeing as how they referenced how the Mane 6 got their cutie marks, I think Starlight might try to stop Rainbow Dash from doing her first Sonic Rainboom, thus making the Mane 6 blank flanks in the present.

Is this what she's going to do or not? Will she be successful? And if she is, what effects will it have besides making the Mane 6 blank flanks?

Posting the rest because others have suggested it as well: With the Mane 6 blank flanks, Twilight will have never become Celestia's student and met up with the other Mane 5. Then Nightmare Moon, Discord, Chrysalis, Sombra, and Tirek would be in power and having turned Equestria into a dark and chatoic wasteland. Seeing how horrible things have turned out without the Mane 6's cutie marks, Starlight reforms and finds a way to restore the timeline. (which would be the ideal of course but I have another, darker theory listed in a comment below, that the timeline won't be restored even if the villains are re-defeated and a whole new timeline is created erasing Episodes 1-115)